CODEWITHSUNDEEP
c++performance
user1899020
user1899020

Reputation: 13575

performance comparison when arguments are pre-known?

I have a function

enum Limit {MIN, MAX};

inline int f(pair<int, int> p, Limit l)
{
    return l == MIN ? p.first : p.second;
}

Compare the computatinal time of f(p, MIN) and p.first, is the first one slower even with a good compiler in release version?

Upvotes: 0

Views: 66

Answers (1)

Richard Hodges
Richard Hodges

Reputation: 69912

Questions of performance are impossible to answer through prediction. It entirely depends on what optimisations the compiler is able to make.

In this contrived case:

#include <utility>

enum Limit {MIN, MAX};

inline int f(std::pair<int, int> p, Limit l)
{
    return l == MIN ? p.first : p.second;
}

extern void emit(int);
int main()
{
  auto p = std::make_pair(4, 5);
  emit(f(p, MIN));
  emit(p.first);
}

There is no difference whatsoever:

main:                                   # @main
        pushq   %rax

        movl    $4, %edi
        callq   emit(int)

        movl    $4, %edi
        callq   emit(int)

        xorl    %eax, %eax
        popq    %rcx
        retq

Upvotes: 1

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